1. Field of the Invention
The present invention relates in general to receiver operation in satellite positioning systems (SATPS) such as the Global Positioning System (GPS), GLONASS, and the proposed Galileo system, and in particular to navigation processing for a SATPS receiver.
2. Description of the Related Art
Navigation processing for SATPS receivers usually comprises processing pseudoranges and Doppler frequency shift measurements to determine position and velocity of the receiver. However, this approach is sometimes unable to distinguish between alternate (valid and invalid) three-satellite solutions. Examples of such processing techniques are found in U.S. Pat. No. 5,416,712 by Geier, U.S. Pat. No. 5,883,595 by Colley, and U.S. Pat. No. 5,590,043 by McBurney, all of which are incorporated by reference herein.
To minimize the limitations in the prior art, and to minimize other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for navigation processing in a satellite positioning system receiver, including methods and apparatus for generating a least-squares fix, checking the validity of measurements, and processing measurements in a low-power mode which involves repeated on/off cycles of the reference oscillator used to make the measurements.
A method in accordance with the present invention comprises separating the three SATPS satellites into a first pair and a second pair, constructing a primary solution and an alternate solution, wherein the primary solution and the alternate solution satisfy the measurement constraints, computing a Doppler difference estimate for the primary solution and a Doppler difference estimate for the alternate solution, computing Doppler difference residuals for the first pair and the second pair of SATPS satellites, and comparing the Doppler difference residuals for said primary and alternate solutions to determine a valid solution. Typically, the computing of a Doppler difference residuals comprises differencing a measured Doppler difference from an estimated Doppler difference for the first pair and the second pair of SATPS satellites. Typical determination of a valid solution comprises comparing the difference between the Doppler difference residuals to a predetermined number. Usually when the Doppler difference residuals exceed the predetermined number, the alternate solution is selected as the valid solution.